Understanding mechanisms of grain refinement of aluminium alloys by inoculation

Abstract

For over half a century, grain refinement of aluminium alloys has been achieved by chemical inoculation; current grain-refinement practice involves the addition of master alloys (e.g. Al – Ti – B, Al – Ti – C) before casting, introducing inoculant particles to the melt. These particles act as nucleation points for α-Al grains, resulting in a uniformly fine, equiaxed as cast microstructure. Despite the ubiquity of this process, its underpinning science was not fully understood, hindering development of the area. From the 1950s onwards, the phase responsible for nucleation in alloys refined by Al – Ti – B was fiercely disputed. The debate focused so closely on this issue that other important factors were frequently ignored. During the 1990s, this debate was resolved through careful thermodynamic reasoning and novel experiments that derived their inspiration far from the foundry. This review focuses on subsequent experimental work and modelling: the expansion of the current understanding of grain refinement to include effects relating to the release of latent heat, the size distribution of inoculant particles and the alloying elements present in the melt (through poisoning and growth-restriction effects). The current contention regarding the nature of the interface between the nucleant phase (TiB₂) and the melt is also discussed. These recent advancements have lead to improvements in grain refining practice resulting in savings in the foundry and development of alternative master alloys for aluminium systems. The ideas have also been successfully applied to other metallic systems, notably magnesium and zinc alloys.     

Relevance of instrumented nano-indentation for the assessment of the mechanical properties of eutectic crystals and α-Al grain in cast aluminum alloys

To better understand the material properties of cast aluminum alloys, the mechanical properties of microscopic structures (the eutectic structure and the α-Al phase) have been systematically examined using a special nano-indentation hardness test machine. In this approach, a triangular indentation is applied directly to the eutectic structure or α-Al phase, and the mechanical properties evaluated through hardness and load-strain relationships. The hardness of the eutectic phase varied, depending on the structural characteristics of the different intermetallic compounds. High values of hardness were obtained structures of the DO₃ type, e.g., Al₅FeSi, but low hardness for CuAl₂ and Mg₂Si. The hardness of CuAl₂ and Mg₂Si had almost the same values as that of the α-Al matrix. In addition, from the nano-indentation hardness test, the effects of α-Al grain characteristics on the mechanical properties were clarified. The hardness of the α-Al grain was linearly related to the grain size and the distance from the grain boundary, where the higher the hardness, the smaller the grain size and the closer to the grain boundary. The hardness level was attributed to the different severity of slip resistance of the atoms during the indentation loading.     

Relationships of diboride phases in Al–Ti(Zr)–B alloys

Abstract

The relationships of diboride phases in Al–Ti(Zr)–B alloys with a variable Ti/B ratio close to the stoichiometry of TiB₂ were studied. The formation of diboride solid solutions was confirmed. A grain refinement mechanism is proposed as that diboride particles in the Al–Ti–B master alloys reacting with aluminium upon adding into an aluminium melt and release titanium into the melt through forming a (Ti,Al)B₂ solid solution and maintain a thin dynamic Ti rich layer on the surfaces of the (Ti,Al)B₂ particles, which nucleates α-Al grains in solidification. The poisoning effect of zirconium on grain refinement of aluminium by Al–Ti–B master alloys is also discussed.     

Influence of melt treatments on sliding wear behavior of Al–7Si and Al–7Si–2.5Cu cast alloys

The microstructures and dry sliding wear behavior of Al–7Si and Al–7Si–2.5Cu cast alloys were studied after various melt treatments like grain refinement and modification. Results indicate that combined grain refined and modified Al–7Si–2.5Cu cast alloys have microstructures consisting of uniformly distributed α-Al grains, eutectic Al– silicon and fine CuAl₂ particles in the interdendritic region. These alloys exhibited better wear resistance in the cast condition compared with the same alloy subjected to only grain refinement or modification. The improved wear resistances of Al–7Si–2.5Cu cast alloys are related to the refinement of the aluminum grain size, uniform distribution of eutectic Al-silicon and fine CuAl₂ particles in the interdendritic region resulting from combined refinement and modification. This paper attempts to investigate the influence of the microstructural changes in the Al–7Si and Al–7Si–2.5Cu cast alloys by grain refinement, modification and combined action of both on the sliding wear behavior.     

Influence of Ti,B and Sr on the microstructure and mechanical properties of A356 alloy

In the present investigation, the microstructural and mechanical properties study of A356 alloy have been discussed. The microstructural aspect of cast A356 alloy employed in the present study is strongly dependent on the grain refinement (Ti and B) and modification (Sr). The mechanical properties such as PS, UTS, %E, %R, YM and VHN have been investigated. This paper deals with the combined effect of grain refinement and modification, which improves the overall mechanical properties of the alloy. It is also a well-known fact that the mechanical properties of cast A356 alloy were improved by subjecting suitable melt treatment such as grain refinement, modification and mould vibration, etc. The quality of castings and their properties can be achieved by refining of α-Al dendrites in A356 alloy by means of the addition of elements such as Ti and B which reduces the size of α-Al dendrites, which otherwise solidifies with coarse columnar α-Al dendritic structure. In addition, modification is normally adopted to achieve improved mechanical properties. Metallographic studies reveal that the structure changes from coarse columnar dendrites to fine equiaxed ones on the addition of grain refiner and further, plate like eutectic silicon to fine particles on addition of 0.20% of Al–10Sr modifier. The present result shows that a reduction in the size of α-Al dendrites, modification of eutectic Si and improvement in the mechanical properties were observed with the addition of grain refiner Al–3Ti, Al–3B and modifier Al–10Sr either individual addition or in combination. The change in the microstructure from coarse columnar α-Al dendrites to fine equiaxed dendrites and plate like eutectic silicon to rounded particles leads to improved mechanical properties.     

Influence of melt treatments and turning inserts on cutting force and surface integrity in turning of Al–7Si and Al–7Si–2.5Cu cast alloys

The microstructures, machinability and surface characteristics of Al–7Si and Al–7Si–2.5Cu cast alloys were studied after various melt treatments like grain refinement and modification. Results indicate that combined grain refined and modified Al–7Si–2.5Cu cast alloys have microstructures consisting of uniformly distributed α-Al grains, eutectic Al–silicon and fine CuAl₂ particles in the interdendritic region. These alloys exhibited better machinability and surface characteristics in the cast condition compared with the same alloy subjected to only grain refinement or modification. Performances of the turning inserts (Un-coated, PVD and Polished CVD diamond coated) were evaluated in machining Al–7Si and Al–7Si–2.5Cu cast alloys under dry environment using a lathe. The Polished CVD diamond coated insert outperformed the Un-coated or PVD-coated cutting inserts which suffered from sizeable edge buildup leading to higher cutting force and poor surface finish. The Polished CVD diamond coated insert shows a very small steady wear without flaking of the diamond film during cutting. This paper attempts to investigate the influence of grain refinement, modification and combined action of both on the microstrutural changes in the Al–7Si and Al–7Si–2.5Cu cast alloys and their machinability and surface finish when different turning inserts used.     

Influence of melt treatments and polished CVD diamond coated insert on cutting force and surface integrity in turning of Al-7Si and Al-7Si-2.5Cu cast alloys

The microstructures, machinability and surface characteristics of Al-7Si and Al-7Si-2.5Cu cast alloys were studied after various melt treatments like grain refinement and modification. The results indicate that combined grain refined and modified Al-7Si-2.5Cu cast alloys have microstructures consisting of uniformly distributed α-Al grains, eutectic Al-silicon and fine CuAl₂ particles in the interdendritic region. These alloys exhibited better machinability and surface characteristics in the cast condition compared with the same alloy subjected to only grain refinement or modification. Performances of the turning inserts (uncoated and polished CVD diamond coated) were evaluated in machining Al-7Si and Al-7Si-2.5Cu cast alloys under dry environment using a lathe. The polished CVD diamond coated insert outperformed the uncoated cutting insert which suffered from sizeable edge buildup leading to higher cutting force and poor surface finish. The polished CVD diamond coated insert shows a very small steady wear without flaking of the diamond film during cutting. This paper attempts to investigate the influence of grain refinement, modification and combined action of both on the microstructural changes in the Al-7Si and Al-7Si-2.5Cu cast alloys and their machinability and surface finish when different turning inserts are used.     

Effect of melt treatment on microstructure and impact properties of Al-7Si and Al-7Si-2.5Cu cast alloys

The microstructures and impact toughness of Al-7Si and Al-7Si-2.5Cu cast alloys were studied after various melt treatments like grain refinement and modification. The results indicate that combined grain refined and modified Al-7Si-2.5Cu alloys have microstructures consisting of uniformly distributed α-Al grains, interdendritic network of fine eutectic silicon and fine CuAl₂ particles in the interdendritic region. These alloys exhibited improved impact toughness in as cast condition when compared to those treated by individual addition of grain refiner or modifier. The improved impact toughness of Al-7Si-2.5Cu alloys are related to breakage of the large aluminum grains and uniform distribution of eutectic silicon and fine CuAl₂ particles in the interdendritic region resulting from combined refinement and modification. This paper attempts to investigate the influence of microstructural changes in the Al-7Si and Al-7Si-2.5Cu cast alloys by grain refinement, modification and combined action of both on the impact toughness.     

The solidification process of Al–Mg–Si alloys

The microstructure and solidification process of three Al–Mg–Si alloys with different magnesium contents have been studied using optical microscopy and the electron probe X-ray microanalysis. The results showed that Al–Mg–Si alloys possessed fairly complicated solidification path: L→α-Al+L1→α-Al+Al15Si2(FeMn)3+L2→α-Al+Al15Si2(FeMn)3+ (α-Al+Mg2Si)+L3→α-Al+Al15Si2(FeMn)3+(α-Al+Mg2Si)+(α-Al+Mg2Si+Al15Si2(FeMn)3), and wide solidification temperature of 75 °C. The magnesium content in the alloys greatly influenced the as-cast microstructure. The higher the magnesium content, the more Mg2Si structure was present. Iron and manganese segregated to the finally solidified zone, which resulted in the formation of ternary eutectic structure. Although their content in the alloys was very low, their effect on solidification behaviour cannot be ignored. This revised version was published online in November 2006 with corrections to the Cover Date.     

Instability of TiC particles in aluminium melts inoculated with an Al-Ti-C grain refiner

Abstract

Microstructural observations on super purity aluminium inoculated with an Al-Ti-C refiner confirm that the nucleant TiC particles present in the refiner are thermodynamically unstable on holding in the melt. The rate of conversion of TiC to Al₄C₃ is characterised using measurements of the titanium content of the aluminium; the conversion is slow enough not to impede grain refinement under normal conditions, but it is strongly temperature dependent. The mechanism of the conversion is examined and shown to be consistent with a progressive rather than a sudden fading of refiner performance. Based on estimates of the evolving size distribution of TiC particles in the melt, grain size predictions are made and shown to be in qualitative agreement with the fading of refinement seen experimentally.     

Precipitation hardening of cast Zr-containing A356 aluminium alloy

The effect of small additions of zirconium on the hardness, grain size, precipitate type and size of cast A356 aluminium alloy was investigated. The cast alloys were solution treated and then artificially aged for different periods of time. Hardness tests and scanning electron microscope (SEM), energy dispersive X-ray (EDX) and X-ray diffraction (XRD) studies were carried out on the as-cast, as-solutionised and age-hardened specimens. Incoherent, coarse Al₃Zr particles formed in the microstructure during the solidification of the alloy and caused grain refinement in the as-cast structure. These particles dissolved and reprecipitated as smaller-size particles during the solution treatment, causing the hardness of the alloy to remain constant at high temperatures for long periods of time due to the slow diffusion of Zr in the α-Al.     

Effect of Sn addition on microstructure and dry sliding wear behaviors of hypereutectic aluminum–silicon alloy A390

In this study, the effect of Sn addition on the microstructure and dry sliding wear behaviors of as-cast and heat-treated hypereutectic A390 alloys was investigated. The microstructural features of the alloys were characterized by means of optical microscope, scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy techniques and their wear characteristics were evaluated at different loads. The worn morphologies of the wear surface were examined by SEM. The results show that the β-Sn in as-cast A390 alloy precipitates mainly in the form of particles within the Al₂Cu network on the interface of the eutectic silicon and α-Al phases and the grain boundaries of α-Al phase. The addition of Sn promotes the disintegrating and spheroidizing of both the eutectic and primary silicon of the A390 alloy during solid solution-aging treatment and β-Sn phase grains coalesces and grows, and some of them form the structure of Sn wrapping Si. The wear rates and friction factors of the as-cast and heat-treated A390 alloys with Sn are lower than those without Sn. At lower load, the addition of Sn changes the wear mechanism of as-cast A390 alloy from the combination of abrasive and adhesive wear without Sn into a single mild abrasion wear with Sn; at higher load, the wear of as-cast A390 alloy without Sn includes abrasion, adhesive, and fatigue one, while the addition of Sn effectively restrains the net-like cracks on the worn surface of the alloy and avoids the fatigue wear emerged.     

Effect of alloying with zinc on SFE of aluminium by study of lattice imperfections in cold worked Al-Zn alloys

A detailed X-ray Fourier line shape analysis has been performed on three compositions of Al-Zn alloys viz. Al-3.55 wt% Zn, Al-14.7 wt% Zn and Al-19.3 wt% Zn infcc phase. It has been found that deformation stacking faults, both intrinsic ά and extrinsic α are absent in the cold worked state and twin fault β is found to be slightly present in the deformed lattice of the two initial compositions of the alloys. Similar to the effect of solute germanium and copper, respectively in Al-Ge and Al-Cu systems, hexagonal zinc also fails to impart faulting infcc Al-Zn system. This corroborates the fact that aluminium has high stacking fault energy.     

Development of titanium alloys by the method of complex alloying

We study the effect of alloying on the mechanical properties of welded joints and base metal of titanium alloys of the system Ti-Al-Mo-V-Cr-Fe. We also formulate the theoretical aspects and principles of complex alloying of titanium alloys and the theory of alloying of additive materials for the welding of α-, (α + β), and β-alloys. It has been shown that (α + β)-(VT23) and β-(VT19) structural titanium alloys, developed on the basis of the proposed theory of complex alloying, provide high weight efficiency of construction of present-day flying vehicles. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 5, pp. 45–50, September–October, 2006.     

Microstructural phase evaluation of high-nitrogen Fe–Cr–Mn alloy powders synthesized by the mechanical alloying process

In this study, the formation of Fe18Cr8MnxN alloys by mechanical alloying (MA) of the elemental powder mixtures was investigated by running the milling process under nitrogen and argon gas atmospheres. The effect of the milling atmosphere on the microstructure and phase contents of the as-milled powders was evaluated by X-ray diffraction and transmission electron microscopy. The thermal behavior of the alloyed powders was also studied by differential scanning calorimetry. The results revealed that in the samples milled under nitrogen, three different phases, namely ferrite (α), austenite (γ), and a considerable amount of amorphous phase are present in the microstructure. In contrast, in the samples milled under argon, the structure contains the dominant crystalline ferrite phase. By progression of MA under the nitrogen atmosphere, the ferrite-to-austenite phase transformation occurs; meanwhile, the quantity and stability of the amorphous phase increase, becoming the dominant phase after 72 h and approaching 83.7 wt% within 144 h. The quantitative results also showed that by increasing the milling time, grain refinement occurs more significantly under the nitrogen atmosphere. It was realized that the infused nitrogen atoms enhance the grain refinement phenomenon and act as the main cause of the amorphization and α-to-γ phase transformation during MA. It was also found out that the dissolved nitrogen atoms suppress the crystallization of the amorphous phase during the heating cycle, thereby improving the thermal stability of the amorphous phase.     

Performance of AlTi5B1, AlTi3B3 and AlB3 master alloys in refining grain structure of aluminium foundry alloys

Abstract

The capacity of AlTi5B1, AlTi3B3 and AlB3 grain refiners to refine the grain structures of AlSi7Mg and AlSi11Cu2 foundry alloys was investigated. The performance of AlTi5B1, well established to be the best grain refiner for wrought aluminium alloys, is not nearly as good with the AlSi7Mg and AlSi11Cu2 alloys. Relatively smaller grains are obtained with the AlTi3B3 grain refiner in both alloys. The AlB3 grain refiner, on the other hand, improves the grain structure only as much as the AlTi5B1 grain refiner does. With as much as 0·04–0·1 wt-%Ti, the commercial alloys cannot enjoy the outstanding potency of the AlB₂ particles since the B supply is readily transformed to TiB₂ particles. However, the grains of the Ti free AlSi7Mg and AlSi11Cu2 alloys (～0·005 wt-%Ti) are very small and nearly globular for the entire range of holding times when inoculated with AlB3, implying not only a remarkable grain refining capacity but also a strong resistance to fading of the grain refinement effect. The lack of Ti in the melt allows the entire B to form AlB₂ particles, the perfect substrates to promote the nucleation of α-Al crystals. Aluminium castings can enjoy grain sizes well below 200 μm, with an addition of 0·02 wt-%B, provided that they are Ti free. That the potent substrates are made available just before the nucleation of α-Al crystals avoids fading effects and is a further advantage of the AlB3 grain refiner in recycling operations.     

Investigation of the effects of microstructure on creep deformation in α2-based titanium aluminide intermetallics

The results of a systematic investigation of the effects of microstructure/substructure on the secondary creep behaviour of α2-based titanium aluminide alloys are presented. This includes a study of the effects of heat treatment on the steady-state creep behaviour of α2+β-processed Ti-24Al-11Nb and Ti-25Al-10Nb-3V-1Mo at 540, 650 and 760°C, and an investigation of the effects of creep deformation on dislocation substructures in Ti-24Al-11Nb. The parameters that control secondary creep deformation are identified for both alloys, and the results are compared with data obtained for conventional high-temperature near-α titanium alloys and β-forged Ti-24Al-10Nb-3V-1Mo. This revised version was published online in November 2006 with corrections to the Cover Date.     

Preparation of Al–Ti–C–Sr master alloys and their refining efficiency on A356 alloy

Al–Ti–C–Sr master alloys with various amounts of Sr were prepared through a method of liquid solidification reactions. The as-prepared Al–Ti–C–Sr master alloys were then used as grain refiners to modify A356 alloy. The microstructures of the Al–5Ti–0.25C–2Sr, Al–5Ti–0.25C–8Sr alloys and modified A356 alloy were investigated. The results showed that the Al–5Ti–0.25C–2Sr alloy consisted of phases of α-Al, lath-shaped or tiny blocky TiAl₃, granular TiC, and blocky or rim AlTiSr, while the Al–5Ti–0.25C–8Sr alloy contained an irregular blocky Al₄Sr phase besides the above-mentioned phases. Satisfactory grain refining and modifying effects were obtained by the addition of Al–Ti–C–Sr alloys (0.5 wt.%) to the A356 alloy. Meanwhile, the sizes of the α-Al dendrites / SDAS(40 µm) decreased to 32.7 µm (or 30 µm).The morphology of eutectic silicon was changed from needle-/platelike form to fibrous/globular form. The grain refinement and modification effects of Al–Ti–C–Sr alloys on A356 alloys were mutually promoted. Compared with the Al–5Ti–0.25C–2Sr alloy, the Al–5Ti–0.25C–8Sr alloy possessed higher efficiency in grain refinement and modification of the A356 alloys.     

The effect of solutes on grain boundary mobility during recrystallization and grain growth in some single-phase aluminium alloys

The effect of solutes (Si, Mn, Mg) in quantities typical of commercial aluminium alloys, on grain boundary mobility in aluminium, has been investigated with in situ annealing and electron backscattered diffraction in the SEM, and grain growth experiments. The in situ experiments provided information on the migration of the high mobility tilt boundaries of misorientations close to 40°〈1 1 1〉. Grain growth experiments were used to investigate boundary migration in alloys of high solute content (1-5wt%Mg), and a comparison between the in situ and bulk experiments is made. The relationship between boundary velocity and driving pressure was found to be linear in all cases, and the activation energies for boundary migration were higher than those controlled by lattice diffusion of the solutes at higher solute concentrations.     

Effect of titanium on the structure and mechanical properties of Cu-Al-Ti alloys

The structure and mechanical properties of Cu10 wt% Al base alloys with 0–2.5 wt% Ti additions were investigated using transmission electron microscopy, optical microscopy and tensile tests. Addition of titanium has a decreasing effect on the grain size after quenching fromα + β region and causes significant strengthening of alloys. Alloy containing 1 wt%Ti quenched from 900° C shows mixture ofα, retainedβ (DO₃), disorderedβ′ (3R) and orderedβ′ ₁ (18R) martensites. Alloy with 2.5 wt% Ti addition after quenching containsα, retainedβ (DO₃), ordered T₁ phase of L2₁ superlattice and orderedβ′ ₁ martensite with either R18 or L1₀ structure indicating different stacking of ordered planes as the effect of titanium addition.